A guy named David Martines announced on YouTube that he'd discovered an "anomaly" which is clearly a secret Martian base, just by using Google maps. You can see what he's calling the base in this image, taken from Google, above.

Said Martines:

I call it Bio-station Alpha, because I'm just assuming that something lives in it or has lived in it. It's very unusual in that it's quite large, it's over 700 feet long and 150 feet wide, it looks like it's a cylinder or made up of cylinders. It could be a power station or it could be a biological containment or it could be a glorified garage - hope it's not a weapon. It could be a way station for weary space travelers. It could also belong to NASA, I don't know that they would admit that.